A composition comprising a radioactive fluorine-labeled amino acid compound is provided, which can be prevented from radiolysis. Disclosed is a composition which comprises a solution containing a radioactive fluorine-labeled amino acid compound as an effective ingredient, in which the pH value of the solution is kept at 2.0-5.9, more preferably 2.0-4.9 in order to inhibit radiolysis. Also, it is possible to further inhibit radiolysis by adding thereto a pharmaceutical additive capable of inhibiting radiolysis, such as a sugar, a sugar alcohol and a sugar lactone, whilst the pH is kept at 2.0-5.9.
